Nature & Environment

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# 2.2 Energy flow in ecosystems

You are about to meet some very large numbers, expressed in scientific notation, and some new units. The new units are those that are used to measure the amount of solar energy received by a part of the Earth's surface. Since plants are dependent on light for photosynthesis, the amount of plant material that can grow in a particular area depends, to a large extent, on how much solar energy reaches it. Energy is measured in joules (J) or, more often, in thousands of joules (kilojoules, kJ). The number of kilojoules received depends on the size of the area and the duration of the period of measurement. If we take an area of one square metre (1 m2) and measure the solar energy received during a whole year, then the units will be kJ per m2 per year, or more scientifically, kJ m−2 year−1. The 'year' is not strictly an SI unit and you may meet several different abbreviations, such as 'yr' (used in this text), 'y', or sometimes 'a', the abbreviation for 'annus' or, when preceded by 'per', 'annum'. Geological time is often quoted in Ma (millions of years).

An enormous amount of solar energy reaches the surface of plants, but most is reflected away and only a small proportion can be used by the plants for photosynthesis. The amount of solar energy falling on typical grassland is 7 × 106kJ m−2 yr−1 (i.e. 7000 000 kJ m−2 yr−1).

## Question 2

Question: Of this large amount of solar energy, only a relatively small amount ends up in plant material - about 85 000 kJ m−2 yr−1. Express this value as a percentage of the total amount of incoming solar energy.